Why particular components of the repertoire develop or fail to develop is a complex problem related to the forces that determine immunodominance at various levels. The NOD mouse has a propensity for generating an autoimmune repertoire and succumbing to Type 1 diabetes (T1D). Nevertheless, there are a variety of ways that the NOD's problems can be readily corrected. We would like to explore some of these pathways that modulate the disease process and involve B cell effects that specifically affect the composition of the T cell repertoire. First, a distinct panel of certain prevalent clonotypes that arise spontaneously in the NOD will be followed throughout the prediabetic and diabetic periods, by using CDR3-length spectroscopy, and following the appearance of "signature peaks" (SPs), each reflecting one of the 3000 possible peaks of CDR3 lengths within BV/BJ gene families. The question of whether there is a clear programmed or random or a simultaneous order of appearance of public clonotypes will be asked. The difference in SPs between NOD mice in B cell sufficient and B cell deficient mice will then be studied. The coincidence of CD4 and CDS clonal appearance will be sought, asking whether it is dependent on the proximity of their specific determinants. With the knowledge of the clonotypic life histories, we will approach two vexing problems. There are data emphasizing the importance of the B cell as an antigen presenting cell in enlisting disease- causing T cells in the NOD: in a B cell knockout mouse, diabetes does not occur, nor can certain relevant Ag-specific responses be made. This suggests that there are great stringencies in the relationships between determinants recognized by the B cell and T cell, and also, that dendritic cells and macrophages cannot generate certain antigenic determinants. We will reconstitute B knockout mice with different populations of B cells to attempt to learn the rules for the stringent requirement for particular B cells. Whether activated B cells are more effective than naive B cells can be discovered with transgenic B cells. Using cyclophosphamide, which removes or inactivates regulatory cells, it will be possible to identify particular SPs as regulatory cells, by following their disappearance in drug-treated mice. Alternatively, certain dominant SPs may be disclosed which are particularly sensitive to regulation. Studies of activation of antigen-specific T cells with B cell Ag-lg constructs will be performed. The mechanism of protection that we will test thoroughly is determinant capture by MHC molecules within B cells acting as APC, and chimeric peptides will be employed that contain the capturing determinant and the determinant to be captured on the same peptide chain or gene segment.